1. Field of the Invention
The present invention concerns a method for electromagnetic pumping by conduction of liquid metals having low electrical conductivity, as well as a device implementing that method.
2. Description of the Prior Art
It is known that in conduction type electromagnetic pumps for liquid or molten metals of the conventional type, an electric current called a conduction current is made to pass in a direction perpendicular to the flux of molten metal to be pumped. At the same time, a magnetic field in a direction which is perpendicular simultaneously to the flux of molten metal and to the electrical conduction current is set up. The result of this according to Laplace's law, is that a force, directed along the axis of the pumping duct in which the molten metal flows, originates in the section of molten metal subjected to the combined action of the magnetic field and of the conduction current.
In alternating current type conduction pumps, the conduction current which crosses the flux of liquid metal is set up in the majority of cases, by means of a conductive spire forming the secondary winding of a transformer, so that very intense currents are thus very easily obtained in that conductive spire. It is therefore indispensable to reduce to the strict minimum the contact resistance through the pumping duct, between the conductive spire and the section of molten metal which the conduction current must cross. A great progress has been made in that field by forming the conductive spire with the molten metal itself and by providing, at the level of the conductive spire, an opening in the duct. In these conditions, the resistance of the circuit formed by the conductive spire is reduced to a very low level each time the metal to be pumped is itself a good electrical conductor, this being the case, more particularly, when molten aluminium is to be pumped. Thus, the use of electrodes in contact with the molten metal flowing in the pumping duct is also avoided.
This method becomes defective, however, when the molten metal to be pumped has low electrical conductivity. Indeed, in that case, losses by Joule effect would be very high. The giving off of heat which would result therefrom would be liable, more particularly, to give rise to the evaporation of the liquid or molten metal and the damaging of the pump. The result of this is that the method of electrical electromagnetic pumping by conduction, improved by the use of a conductive spire of liquid metal could not be applied without modification to the pumping of liquid or molten metals having low electrical conductivity.
One of the aims of the present invention has been to improve a very simple method making it possible to use a conductive spire of molten metal to feed the active section of the pumping duct with current while keeping the resistance of the circuit formed by the spire within acceptable limits.